CVE-2026-35209

defu is software that allows uers to assign default properties recursively. Prior to version 6.1.5, applications that pass unsanitized user input (e.g. parsed JSON request bodies, database records, or config files from untrusted sources) as the first argument to `defu()` are vulnerable to prototype pollution. A crafted payload containing a `__proto__` key can override intended default values in the merged resul. The internal `_defu` function used `Object.assign({}, defaults)` to copy the defaults object. `Object.assign` invokes the `__proto__` setter, which replaces the resulting object's `[[Prototype]]` with attacker-controlled values. Properties inherited from the polluted prototype then bypass the existing `__proto__` key guard in the `for...in` loop and land in the final result. Version 6.1.5 replaces `Object.assign({}, defaults)` with object spread (`{ ...defaults }`), which uses `[[DefineOwnProperty]]` and does not invoke the `__proto__` setter.

CVSS v3 7.5 HIGH

7.5^/10

CVSS v3 : HIGH

Vector :

Exploitability : 3.9 / Impact : 3.6

Attack Vector NETWORK

Attack Complexity LOW

Privileges Required NONE

User Interaction NONE

Confidentiality Impact NONE

Integrity Impact HIGH

Availability Impact NONE

Scope UNCHANGED

References

Link	Resource
https://github.com/unjs/defu/commit/3942bfbbcaa72084bd4284846c83bd61ed7c8b29
https://github.com/unjs/defu/pull/156
https://github.com/unjs/defu/releases/tag/v6.1.5
https://github.com/unjs/defu/security/advisories/GHSA-737v-mqg7-c878

Configurations

No configuration.

History

06 Apr 2026, 18:16

Type	Values Removed	Values Added
New CVE

Information

Published : 2026-04-06 18:16

Updated : 2026-04-07 13:20

NVD link : CVE-2026-35209

Mitre link : CVE-2026-35209

CVE.ORG link : CVE-2026-35209

JSON object : View

Products Affected

No product.

CWE

CWE-1321

Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

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